as far as it going dead most people dont realize how long it takes to charge your battery back up after a restart, at idle it would likely take an hour or more. so you were running it down with the stereo, and running it down alot more each time you start it. if you want to run the radio with out the engine you really need a separate deep cycle type battery, second to that would be use it for a while and only try to start it before you are going to go somewhere because you may only have one start left in it.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock